Light emitting diode (LED) provides many advantages such as a longer lifespan, less power consumption, higher illumination and more eco-friendly materials. With advance of LED fabrication process and lower cost, applications of LED also have been greatly expanded, in addition to being adopted on traffic lights or indication light signals of electric appliances, it also can be used on environmental decoration, lighting fixtures and the like. In order to allow the LED to be adapted to the general light bulbs, many techniques have been proposed in prior arts to couple the LED with conventional lamp shell. For instance, R.O.C. patent No. I293807 entitled “LED light bulb equipped with a constant current circuit” discloses an LED light bulb that includes a lamp cap, a lamp shell, a plurality of LEDs coupled in series and a step-down constant current circuit. The lamp cap has electrodes connected to a power source. The LEDs are connected to the step-down constant current circuit to receive a constant current to let the LEDs emit light. The LED light bulb thus formed can be directly mounted onto a conventional lamp socket for use. But driving the LEDs requires a stable DC current. The driving circuit to convert commercial power to DC generates and accumulates waste heat constantly during conversion. Moreover, DC passing through the impedance of the LEDs also generates a lot of waste heat. All those waste heat produces excessive high temperature and could damage the LEDs or driving circuit after used for a long duration, or lower the lifespan. To remedy these problems, many types of LED light bulbs equipped with a heat dissipation structure have been developed in the industry. For instance, R.O.C. patent No. M358247 entitled “LED lamp adaptable to varying lamp sockets” discloses an LED heat dissipation module with a coupling end having an opening and an annular latch groove formed on the periphery of the opening to couple with a corresponding lamp socket. The opening has another end coupled with an LED lighting element. The LED lighting element is electrically connected to the lamp socket. The LED heat dissipation module is located on the peripheries of the LED lighting element and lamp socket to conduct heat. Other references of LED light bulbs adopting similar metal heat sink can be found in R.O.C. patent Nos. M345944 and M381743, and R.O.C. publication No. 201020457.
All the aforesaid references use a metal heat sink (mostly aluminum heat sink) to enhance heat dissipation to allow the light bulb to receive greater power without being burned. However, the conventional LED light bulbs mostly adopt a RC drop passive power source (called “passive power source” in short hereinafter) incorporating with a rectification circuit to supply power. The passive power source has poor efficiency during greater power output. To increase the power, the size of the circuit also has to be increased. The reason why the conventional LED light bulbs adopt the passive power source is because it is easier to pass safety test).
To further improve the power of the LED light bulbs, using a switch-type power source is a preferred choice to get better power usage efficiency. But using the switch-type power source has to pass rigorous safety test, including high voltage applying test, in which a voltage approaching four thousand volts is applied around the light bulb, and insulation between the switch-type power source in the light bulb and the applied external voltage has to be securely in place. In the conventional techniques adopting the passive power source, the passive power source usually is covered by a shrinking film and injected with adhesive for positioning. The adhesive also can enhance insulation effect. But test results prove that the switch-type power source covered by the shrinking film and encased by the adhesive cannot pass the high voltage applying test.
To enhance the power to meet requirements, adopting the switch-type power source for power supply is a preferable choice, but it must overcome the barrier of passing the safety test. In addition, the LED light bulb with a greater power also have to resolve the heat dissipating problem, hence a metal heat sink is usually required. This creates the difficulty in passing the safety test. Thus these two contradictory factors create greater difficulty in design in the industry.